Industrial processing plants use control valves in a wide variety of applications such as, for example, controlling product flow in a food processing plant, maintaining fluid levels in large tank farms, etc. Automated control valves are used to manage the product flow or to maintain the fluid levels by functioning like a variable passage. The amount of fluid flowing through a valve body of the automated control valve can be accurately controlled by precise movement of a valve member (e.g., a plug). The control valve or its valve member may be accurately controlled via an actuator and a remotely operated instrument or valve controller, which communicates with a process control computer or unit to receive commands from the process control unit and position the valve member to change the fluid flow through the control valve. Typically, a position sensor within the control valve facilitates accurate positioning of the valve member and, thus, accurate process control.
When the process control computer issues a command to change the flow through the control valve, the valve controller typically determines the present position of the valve member and applies appropriate corrective action via the actuator to position the valve member as commanded by the process control computer. Some actuators are driven by a pressurized air source, which is controlled by the valve controller. For example, in a spring and diaphragm actuator operating a sliding-stem valve, variations in air pressure applied to a large diaphragm displace the diaphragm and the valve member, which is coupled to the diaphragm. Thus, by changing the air pressure applied to the diaphragm, the valve controller can modify the position of the valve member and control fluid flow through the control valve. To properly control the fluid flow, the valve controller typically monitors the current position of the valve member and the position to which the valve member must be displaced in response to a new command signal. A position sensor is typically disposed between the valve controller and the actuator stem of the sliding-stem valve. The output of the position sensor may be communicated directly to the valve controller to provide stem position data for control of the valve member.
Some known position sensors, such as potentiometers, require dynamic or moving mechanical linkages to communicate movement of the valve member to the position sensor. However, manufacturers have developed non-contacting position sensors to improve sensor reliability. One type of non-contacting position sensor is a magnetic position sensor. Magnetic position sensors detect movement or displacement between two members by attaching a magnetic flux source, typically a magnet, to a first member and a sensor, such as a Hall Effect sensor, to a second member. The magnetic flux source provides a magnetic field that is detected by the sensor. Movement by one or both of the first and second members produces relative displacement to cause a different portion of the magnetic field to be detected by the sensor, thereby changing the output of the sensor. This output can be related directly to the relative displacement between the actuator and the valve stem.
Non-contacting position sensors are adaptable and can measure various forms of displacement. However, replacing a mechanical linkage position sensor with a non-contacting position sensor may be limited by the method of attaching the non-contacting position sensor to the actuator, and by the number of magnets required for the amount of displacement to be measured. For example, a non-contacting position sensor may require the development of a different mounting bracket or a housing for each type of actuator to which the non-contacting position sensor is to be attached.
FIG. 1 is a partially cut-away schematic illustration of a known mechanical linkage position sensor 10 mounted on an end-mount rotary actuator 60. The position sensor 10 includes a feedback arm assembly 12 having a feedback arm 14, a roller 15, an axle 16, a feedback arm torsion spring 17, a spring connector arm 18, a bias spring 19, an extension arm 20 having a slot 22, and a sensor assembly 30. The sensor assembly 30 includes an arm 32 connected to a potentiometer 34 and to the bias spring 19, and a pin 36 extending from the arm 32 and received in the slot 22. The position sensor 10 is contained in a housing 40 that includes a mounting adapter 42 and a mounting bracket 44. The mounting bracket 44 has an axle housing 46 that extends laterally to receive rotatably the axle 16. Additionally, valve controller 50 is mounted to the mounting bracket 44 of the housing 40.
The rotary actuator 60 includes a rotatable actuator shaft 62 displaceable by a movable valve stem 64. The rotatable actuator shaft 62 includes a sloped surface cam member 66 engaged by the roller 15 of the position sensor 10. A valve member (not-shown) is operated by the rotatable actuator shaft 62 to control flow through the valve member.
During the operation of the rotary actuator 60 illustrated in FIG. 1, a command signal from a process control computer or unit (not shown) is communicated to the valve controller 50, which operates the rotary actuator 60. The operation of the rotary actuator 60 causes the movable valve stem 64 to move downwardly to rotate the rotatable actuator shaft 62, sloped surface cam member 66, and the valve member (not shown). The roller 15 and the feedback arm 14 pivot about the axle 16 such that the extension arm 20 and the slot 22 cause the pin 36 and the arm 32 to move and operate the potentiometer 34. The potentiometer 34 communicates an electrical signal (e.g., a changing resistance value) to the valve controller 50. The electrical signal is related to the position of the rotatable actuator shaft 62 and the valve member so that the process control computer can determine the position of the valve member and apply any appropriate corrective action or a new command signal through the valve controller 50 and the rotary actuator 60.
When used in end-mount rotary actuators or remote mount feedback units, the mechanical linkage of the position sensor 10 illustrated in FIG. 1 may be subjected to rugged service conditions. The bias spring 19 exerts considerable force on the arm 32 and the pin 36, whereby during rugged service conditions it is possible for the pin 36 to be sheared off by the extension arm 20. Likewise, other wear points can develop within the mechanical linkage of the position sensor 10 and cause the movable valve stem 64 to become disconnected from the valve controller 50.
Long displacement or long-stroke actuators tend to have parts that rotate and vibrate more than the parts of short-stroke actuators and, thus, present alignment and vibration problems for non-contacting position sensors. A known non-contacting position sensor for a short-stroke actuator requires a large number of magnets in the array of magnets. The use of a non-contacting position sensor configured for a short-stroke actuator with a long-stroke actuator, or an end mount rotary actuator, or a remote mount feedback unit, may require a relatively large number of magnets to measure displacement. A non-contacting position sensor having such a relatively large number of magnets may be expensive and may require long lead times to manufacture.
FIG. 2 is a partial cut-away, schematic illustration of a known position sensor 80 mounted on a portion of a long-stroke sliding-stem actuator 70. The sliding-stem actuator 70 includes a movable valve stem 74 having a ramped or sloped surface cam member 76. A valve member (not shown) is operated by the movable valve stem 74 to control flow through the valve member. The ramped or sloped surface cam member 76 is slidably engaged by a roller 85 mounted on a feedback arm 86, which is pivotally coupled to an axle 88 of the position sensor 80. The position sensor 80 includes a mechanical linkage assembly and potentiometer similar to the linkage assembly and the potentiometer of the position sensor 10 illustrated in FIG. 1, and thus need not be described in further detail herein. The position sensor 80 is held by a mounting bracket 90 to which a valve controller 95 is attached.
The long-stroke sliding-stem actuator 70 and the position sensor 80 operate similar to the operation described above in connection with the rotary actuator 60 and the position sensor 10 illustrated in FIG. 1. A command signal from a process control computer (not shown) is communicated to the valve controller 95, which operates the actuator 70. The operation of the actuator 70 causes the movable valve stem 74 to move downwardly to operate the valve member and to displace the ramped or sloped surface cam member 76. In response to the movement of the ramped or sloped surface cam member 76, the roller 85 and the feedback arm 86 pivot about the axle 88 to operate the position sensor 80. The position sensor 80 communicates an electrical signal to the valve controller 95, which communicates with the process control computer. In this manner, the electrical signal is related to the position of the movable valve stem 74 and the valve member so that the process control computer can determine the position of the valve member and apply any appropriate corrective action or a new command signal through the valve controller 95 and the long-stroke sliding-stem actuator 70.
The use of a known non-contacting position sensor in place of a mechanical linkage position sensor in a long-stroke actuator such as, for example, the position sensor 80 of the example long-stroke sliding-stem actuator 70, would require substantial redesign and development to resolve scale-up issues. For example, the large rotational forces imposed by long-stroke actuators to their structural members tend to break off a sensing fork of a directly connected non-contacting position sensor. Also, a large number of magnets can be required to measure the stroke of a long-stroke actuator (typically four magnets per inch of stroke). Thus, new magnet arrays would have to be developed for actuators having strokes as long as twelve to twenty-four inches in length. Additionally, new mounting adapters and plates would have to be designed to enable the non-contacting position sensor to be mounted to an actuator.